/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 */
package org.apache.iotdb.db.wal.io;

import org.apache.iotdb.db.wal.buffer.WALEntry;

import java.io.Closeable;
import java.io.File;
import java.io.IOException;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.file.StandardOpenOption;
import java.util.Iterator;

import static org.apache.iotdb.db.wal.io.WALWriter.MAGIC_STRING;
import static org.apache.iotdb.db.wal.io.WALWriter.MAGIC_STRING_BYTES;

/**
 * This reader returns {@link WALEntry} as {@link ByteBuffer}, the usage of WALByteBufReader is like
 * {@link Iterator}.
 */
public class WALByteBufReader implements Closeable {
    private final File logFile;
    private final FileChannel channel;
    private final WALMetaData metaData;
    private final Iterator<Integer> sizeIterator;

    public WALByteBufReader(File logFile) throws IOException {
        this.logFile = logFile;
        this.channel = FileChannel.open(logFile.toPath(), StandardOpenOption.READ);
        if (channel.size() < MAGIC_STRING_BYTES || !readTailMagic().equals(MAGIC_STRING)) {
            throw new IOException(String.format("Broken wal file %s", logFile));
        }
        // load metadata size
        ByteBuffer metadataSizeBuf = ByteBuffer.allocate(Integer.BYTES);
        long position = channel.size() - MAGIC_STRING_BYTES - Integer.BYTES;
        channel.read(metadataSizeBuf, position);
        metadataSizeBuf.flip();
        // load metadata
        int metadataSize = metadataSizeBuf.getInt();
        ByteBuffer metadataBuf = ByteBuffer.allocate(metadataSize);
        channel.read(metadataBuf, position - metadataSize);
        metadataBuf.flip();
        metaData = WALMetaData.deserialize(metadataBuf);
        // init iterator
        sizeIterator = metaData.getBuffersSize().iterator();
        channel.position(0);
    }

    /**
     * Like {@link Iterator#hasNext()}
     */
    public boolean hasNext() {
        return sizeIterator.hasNext();
    }

    /**
     * Like {@link Iterator#next()}
     */
    public ByteBuffer next() throws IOException {
        int size = sizeIterator.next();
        ByteBuffer buffer = ByteBuffer.allocate(size);
        channel.read(buffer);
        buffer.clear();
        return buffer;
    }

    private String readTailMagic() throws IOException {
        ByteBuffer magicStringBytes = ByteBuffer.allocate(MAGIC_STRING_BYTES);
        channel.read(magicStringBytes, channel.size() - MAGIC_STRING_BYTES);
        magicStringBytes.flip();
        return new String(magicStringBytes.array());
    }

    @Override
    public void close() throws IOException {
        channel.close();
    }

    public long getFirstSearchIndex() {
        return metaData.getFirstSearchIndex();
    }
}
